From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 47787C4338F for ; Thu, 5 Aug 2021 21:08:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 227A8601FE for ; Thu, 5 Aug 2021 21:08:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241754AbhHEVIl (ORCPT ); Thu, 5 Aug 2021 17:08:41 -0400 Received: from mail.kernel.org ([198.145.29.99]:58912 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241387AbhHEVIj (ORCPT ); Thu, 5 Aug 2021 17:08:39 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 71EF260EBC; Thu, 5 Aug 2021 21:08:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1628197704; bh=SRtaowijoG8twAxHHQPP1frqmBFYAiBOGw+39md1RVo=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=nHRGhN7ts/aOENkjY13K+SLhMIvNR351kTPTtkF6AnSvp5rr/ExKU5kckq0dKRbpY O6+vZb5T1E5OZ/iJVTPbClsqti5PgCkvIULQYEfLc0ffrXYE6czPD2xZPkOCzDu0SV ehNOYEfQBriqlXvRGwYghWT9mMTFacUzReWgtgHw1jGf1UAvE7bUBwx9WqPmgffoPa i9djO3u2yhnd6R7dbjnJz8WaI/kVG/o44IsEbL2vTumJaocbnWNIcQFwHTL6F6bm2b 3+R3xcp+yWN5TVVD8yR9/UpOnGZCf9kcbEu5i/Sb0t0eoF37mJWDuGa9wfdujoTxhR 9Ih14r7LrNMmQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 3D48D5C271F; Thu, 5 Aug 2021 14:08:24 -0700 (PDT) Date: Thu, 5 Aug 2021 14:08:24 -0700 From: "Paul E. McKenney" To: Juri Lelli Cc: josh@joshtriplett.org, linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org, peterz@infradead.org, bigeasy@linutronix.de, tglx@linutronix.de, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, joel@joelfernandes.org, rcu@vger.kernel.org Subject: Re: [PATCH] rcu: Make rcu_normal_after_boot writable on RT Message-ID: <20210805210824.GA1203206@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210805080123.16320-1-juri.lelli@redhat.com> <20210805160337.GI4397@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210805160337.GI4397@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 05, 2021 at 09:03:37AM -0700, Paul E. McKenney wrote: > On Thu, Aug 05, 2021 at 10:01:23AM +0200, Juri Lelli wrote: > > Certain configurations (e.g., systems that make heavy use of netns) > > need to use synchronize_rcu_expedited() to service RCU grace periods > > even after boot. > > > > Even though synchronize_rcu_expedited() has been traditionally > > considered harmful for RT for the heavy use of IPIs, it is perfectly > > usable under certain conditions (e.g. nohz_full). > > > > Make rcupdate.rcu_normal_after_boot= again writeable on RT, but keep > > its default value to 1 (enabled) to avoid regressions. Users who need > > synchronize_rcu_expedited() will boot with rcupdate.rcu_normal_after_ > > boot=0 in the kernel cmdline. > > > > Signed-off-by: Juri Lelli > > Makes sense to me! > > But would another of the -rt people be willing to give an Acked-by? > For example, maybe they would prefer this kernel boot parameter to be > exposed only if (!PREEMPT_RT || NO_HZ_FULL). Or are there !NO_HZ_FULL > situations where rcu_normal_after_boot makes sense? Ah, and this will also need to be reflected in the WARN_ON_ONCE() in synchronize_rcu_expedited_wait() in kernel/rcu/tree_exp.h. Thanx, Paul > > --- > > kernel/rcu/update.c | 2 -- > > 1 file changed, 2 deletions(-) > > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > > index c21b38cc25e9..0fdbf937edac 100644 > > --- a/kernel/rcu/update.c > > +++ b/kernel/rcu/update.c > > @@ -57,9 +57,7 @@ > > module_param(rcu_expedited, int, 0); > > module_param(rcu_normal, int, 0); > > static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT); > > -#ifndef CONFIG_PREEMPT_RT > > module_param(rcu_normal_after_boot, int, 0); > > -#endif > > #endif /* #ifndef CONFIG_TINY_RCU */ > > > > #ifdef CONFIG_DEBUG_LOCK_ALLOC > > -- > > 2.31.1 > >